Proxy of routing protocols to redundant controllers

ABSTRACT

In an embodiment, a method exchanges routing data within a network including control and forwarding on separate devices. In the method, a first reachability information session is established between a forwarding device and a first control device and a second reachability information session is established between the forwarding device and a second control device. Also, a third reachability information session is established between the forwarding device and an external routing device. Finally, advertisements are exchanged between the first and second reachability information sessions and the third reachability information session such that the first and second control device appear to the external routing device to be a single device. System and computer program product embodiments are also disclosed.

BACKGROUND

1. Field

This application is generally related to network routing.

2. Related Art

A communication network may, for example, provide a network connectionthat allows data to be transferred between two geographically remotelocations. A network connection may span multiple links connectingcommunication devices such as routers. Networks may have differenttopologies depending on how the links are interconnected throughcommunication devices. Given a particular network topology, multipleroutes may be available between a source and destination. Some routesmay be more desirable than others depending on current capacity andusage.

Traditional routing algorithms rely on local information each router hasfrom its neighboring links and devices to route data. A router maintainssuch information in a routing table. The routing table has entriesdesignating a next hop for various destination addresses, or groups ofdestination addresses. Based on the destination address of an incomingpacket, a router uses its routing table to forward the packet to aspecific neighboring device. To develop the routing table, each routeruses a protocol like Border Gateway Protocol (BGP) to exchange routingand reachability information with local neighboring routers. In thisway, each router both forwards packets and conducts control functions toupdate its own routing table.

While using local information may be desirable in some contexts, it maynot always route data efficiently. To route data more efficiently,another technique, referred to as Software Defined Networks (SDNs),separates the control and forwarding functions into separate devices.The control device uses a global knowledge of the network topology todetermine a path through the network of forwarding devices forindividual data flows. In this way, the routing control device may, forexample, establish paths that minimize delay or maximize bandwidththrough the network.

BRIEF SUMMARY

In an embodiment, a method exchanges routing data within a networkincluding control and forwarding on separate devices. In the method, afirst reachability information session is established between aforwarding device and a first control device, and a second reachabilityinformation session is established between the forwarding device and asecond control device. The first and second reachability informationsessions enable the respective first and second control devices to sendadvertisements indicating what addresses are reachable through thenetwork. Also, a third reachability information session is establishedbetween the forwarding device and an external routing device. The thirdreachability information session enables the forwarding device toexchange advertisements with the external routing device indicating whataddresses are reachable through the respective forwarding and externalrouting devices. Finally, advertisements are exchanged between the firstand second reachability information sessions and the third reachabilityinformation session such that the first and second control device appearto the external routing device to be a single device.

System and computer program product embodiments are also disclosed.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments, are describedin detail below with reference to accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present disclosure and, togetherwith the description, further serve to explain the principles of thedisclosure and to enable a person skilled in the relevant art to makeand use the disclosure.

FIG. 1A is a diagram of a network having multiple, redundant controldevices that are separate from the forwarding devices.

FIG. 1B is a diagram of a network having a local termination module thatmasks the multiple control devices to an external network.

FIGS. 2A-B are diagrams illustrating an example operation of the systemin FIG. 1.

FIG. 3 is a flowchart of a method for masking redundant controllers,according to an embodiment.

FIG. 4 is a diagram showing the system of FIG. 1 in greater detail.

The drawing in which an element first appears is typically indicated bythe leftmost digit or digits in the corresponding reference number. Inthe drawings, like reference numbers may indicate identical orfunctionally similar elements.

DETAILED DESCRIPTION

As described above, having a control device separate from the forwardingdevices make routing decision can enable more intelligent routingdecisions on a service provider network. But having a single controldevice creates a single point of failure. If that control device fails,no updates could be made to any of the routing tables on the forwardingdevices. To deal with this, embodiments have multiple, redundant controldevices.

But, having multiple control devices on a service provider network canmake configuration difficult for users. In particular, the customer mayhave to configure its routers on its network to accept reachabilityinformation from multiple different control devices. This may involveconfiguring its routers to operate with multiple reachabilityinformation sessions, such as Border Gateway Protocol (BGP) sessions,that each provide reachability information to the customer routers. Thisconfiguration may be difficult. And having multiple reachabilityinformation sessions to the customer network may require transmission ofextra traffic and involve extra processing overhead on the customerrouters. While aspects are described with respect to customer networksand customer routing devices for illustrative purposes, a skilledartisan would recognize that embodiments apply to any external network,that is, any network external to the service provider network.

To deal with these issues of multiple control devices, embodimentsprovide an additional module on a forwarding device at the edge of theservice provider network, where the service provider network interfaceswith the customer network. The module masks the existence of themultiple control devices, making it appear to the customer that only asingle control device is being used.

Specifically, the module establishes reachability information sessions,such as BGP sessions, with the customer routers and with each controldevice in the service provider network. The module exchanges routingdata, such as route advertisements, obtained from the customerequipment's reachability information session and the control device'sreachability information sessions. When the module receives a routeadvertisement from the customer equipment, it forwards it onto each ofthe control devices. And, when the control devices broadcast routeadvertisements, the module forwards only the first-received one onto thecustomer routing equipment. These and other embodiments are describedbelow with respect to the accompanying figures.

FIG. 1A is a diagram of a system 100 having multiple, redundant controldevices that are separate from the forwarding devices.

System 100 includes a service provider network 120 and a customernetwork 130. Service provider network 120 may be a metropolitan areanetwork (MAN) or wide area network (WAN) that connects at least twogeographically disparate locations. Customer network 130, on the otherhand, may be a local area network that, for example, connects differentcomputers within a single entity or building.

Service provider network 120 includes a plurality of forwarding devices:forwarding devices 106, 108, and 110. Each forwarding device may have aplurality of ports and forward packets of data from one port to another.To forward the data, each forwarding device may have a routing table andmay forward information according to information in its routing table.Specifically, the routing table may map particular addresses or subnetsto particular output ports. When the forwarding device receives a packetof data, the forwarding device examines the packet's destination addressto identify an entry in the routing table. In addition to examining thepacket's destination address, the forwarding device examines any labelsassociated with packet, such as Multiprotocol Label Switching (MPLS)labels, to identify the entry in the routing table. That entry in therouting table specifies which port on the forwarding device to forwardthe packet.

Instead of configuring their own routing tables, in a software definednetworks as disclosed herein, the forwarding devices' routing tables areconfigured by control devices. FIG. 1A depicts two control devices:control devices 102 and 104. Control devices 102 and 104 each maytransmit information to forwarding devices 106, 108, and 110 toconfigure their routing tables. Control devices 102 and 104 mayconfigure the routing tables to route data from a particular data flowalong a particular path.

In embodiments, control devices 102 and 104 may be redundant. Eachcontrol device may have identical, or mirror image, information aboutthe topology of service provider network 120 and may be able todetermine identical paths through service provider network 120independently of one another. By being redundant, if one of the controldevices goes down, the other may continue to configure all of theforwarding devices on service provider network 120. While forillustration only two control devices are shown in FIG. 1A, a skilledartisan would recognize that additional control devices may be used andadding additional control devices would add additional redundancy tosystem 100.

In addition to configuring the routing tables of the forwarding devicesin service provider network 120, control devices 102 and 104 may eachestablish respective reachability information sessions 120 and 122 withat least one device on customer network 130. The reachabilityinformation sessions may exchange routing and reachability informationbetween service provider network 120 and the devices on customer network130. Border gateway protocol (BGP) is a common type of reachabilityinformation session protocol, but other types of reachabilityinformation session protocols may be used, for example Open ShortestPath First (OSPF) and Intermediate System to Intermediate System (ISIS).

Control devices 102 and 104 may use the information received fromcustomer network 130 over reachability information sessions 120 and 122respectively to develop their knowledge of the global network topology.For example, customer network 130 may inform control devices 102 and104, over reachability information sessions 120 and 122, that itincludes or can reach a particular set of destination addresses (such asan IP address subnet). Based on this information, control devices 102and 104 may update their respective models of the global networktopology. And control devices 102 and 104 may use the updatedinformation to determine the routing tables for forwarding devices106-110.

The routing devices on customer network 130 may also use the informationreceived over reachability information sessions 120 and 122 to configuretheir routing tables. Customer network 130 may include separate controland forwarding devices as similar to service provider network 120, orcustomer network 130 may include routers and switches that both forwarddata, and control and calculate their own routing tables. Either way,the routing devices on customer network 130 use the reachabilityinformation received from reachability information sessions 120 and 122to configure their routing tables. They configure their routing tablessuch that data destined for an address reachable through serviceprovider network 120 can be forwarded to service provider network 120.

Having multiple control devices in this way may add to redundancy. Buthaving multiple reachability information sessions 120 and 122 mayincrease the burden on the customer of configuring devices on customernetwork 130. Specifically, a customer may have to configure its devicesto operate with both reachability information session 120 and 122. Foran administrator of customer network 130 that may be unfamiliar withservice provider network 120, this configuration can be burdensome. Todeal with this issue, embodiments aggregate data from the differentcontrol devices 102 and 104 to make for a single reachabilityinformation session with customer network 130 as illustrated in FIG. 1B.

FIG. 1B illustrates a system 150 that has a local termination module 170that masks the multiple control devices to a customer network. Likesystem 100 in FIG. 1A, system 150 has a customer network 130, whichincludes a customer routing device 180, and a service provider network120, which in turn has two control devices (102 and 104) and threeforwarding devices (106, 108, and 110).

Some forwarding devices reside on an edge of service provider network120, meaning that they directly connect to an outside network. Accordingto embodiment, it is these edge forwarding devices that include a localtermination module. In system 150, forwarding device 108 is an edgeforwarding device, because it connects with customer network 130.Accordingly, forwarding device 108 includes local termination module170.

Local termination module 170 establishes a reachability informationsession with the external network that its forwarding device isconnected to and with each control device on service provider network120. Here, local termination module 170 establishes a reachabilityinformation session 164 with customer network 130, and reachabilityinformation sessions 160 and 162 with control devices 102 and 104respectively. Local termination module 170 acts as a proxy for thereachability information sessions exchanging messages between them.

In particular, local termination module 170 exchanges messages betweenreachability information sessions 160, 162, and 164 to make the multiplecontrol devices 102 and 104 appear to customer routing device 180 to bea single device. For example, local termination module 170 may receivean advertisement from customer routing device 180 via the reachabilityinformation session 164. The advertisement may include reachabilityinformation, and the reachability information may indicate whichaddresses customer routing device 180 can forward information receivedfrom service provider network 120 to. When local termination module 170receives the advertisement, it sends the enclosed reachabilityinformation to the control devices 102 and 104 via the reachabilityinformation sessions 160 and 162. Using the reachability information,control devices 102 and 104 may update their knowledge of the networktopology. And control devices 102 and 104 use the updated networktopology to determine routing tables for forwarding devices 106, 108,and 110 with respect to those addresses in the reachability information.Specifically, control devices 102 and 104 may add entries to the routingtables to route data destined for addresses within customer network 130,and reachable from customer network 130, toward customer network 130.Then, control devices 102 and 104 transmit the updated routing tables toforwarding devices 106, 108, and 110, enabling forwarding devices 106,108, and 110 to route data to customer network 130.

Because local termination module 170 has reachability informationsessions 160 and 162 with both control device 102 and 104, it willreceive reachability information from both. And because control device102 and 104 are redundant, and potentially mirror images of each other,the information received from control devices 102 and 104 will beduplicative. When duplicative information is received, local terminationmodule 170 may only forward on the first-received information. Todetermine whether reachability information has been previously sent,local termination module 170 may store the reachability information, oran identification of it such as a hash. When local termination module170 receives reachability information from a control device, it maycheck the storage to determine whether the reachability information hasbeen previously forwarded to the customer routing device. Then,depending on the check, it may forward the information.

In an example operation, local termination module 170 may receive anadvertisement from control device 102 via reachability informationsession 160. The advertisement may include reachability information thatin turn includes a number of entries. Each entry may map destinationaddresses reachable through service provider network 120 to a next hopaddress. When local termination module 170 receives the advertisement,local termination module 170 may check its local storage to determinewhether it has already sent. If it has already been sent, localtermination module 170 does nothing. If it has not already been sent,local termination module 170 may forward on the data to customer routingdevice 180.

FIGS. 2A-B are diagrams showing a system 200 that illustrates howreachability information is propagated from one network to another. Inaddition to the components of system 150, system 200 includes a customernetwork 230 that includes a routing device 204. Routing device 204 isconnected to the service provider network at forwarding device 110. Insystem 200, forwarding device 110, being an edge forwarding device,includes a local termination module 220.

As illustrated in FIG. 2A, customer network 130 has an address space of20.0.0.0/24. It has been assigned the IP addresses within that space,and so any packets addressed to an IP address whose first three bytesrepresent 20.0.0 should be routed to customer network 130. Similarly,customer network 230 has an address space of 30.0.0.0/24. It too hasbeen assigned those IP addresses, and so any packets addressed to an IPaddress whose first three bytes represent 30.0.0 should be routed tocustomer network 230.

When customer network 130's routing device 180 is attached to forwardingdevice 108, it sends an advertisement message 210. Advertisement message210 includes reachability information indicating that the 20.0.0.0/24subnet is addressable through customer routing device 180.

Forwarding device 108 receives advertisement message 210 and forwardsits reachability information onto both control devices in two separaterouting messages: routing messages 212 and 214 for control devices 102and 104 respectively. Control devices 102 and 104 update their model ofthe global network topology according to the reachability information inmessages 212 and 214. As described above with respect to FIG. 1B, ituses this information to update the routing tables of forwarding devices106, 108, and 110 to route traffic addressed to 20.0.0.0/24 to customerrouting device 180. In addition to updating the routing tables of thedevices on the service provider network, control devices 102 and 104 canalso send routing advertisements to other external networks asillustrated in FIG. 2B.

FIG. 2B illustrates how control devices 102 and 104 advertisereachability information to customer network 230. As mentioned above,control devices 104 have each updated their models of the global networktopology to reflect the fact that 20.0.0.0/24 is reachable throughcustomer network 130, which is connected to the service provider networkat forwarding device 108. In FIG. 2B, control devices 102 and 104advertise to customer network 230 that 20.0.0.0/24 is reachable throughthe service provider network.

Control devices 102 and 104, having both updated their topology models,each send a respective advertisement 260 and 262 to forwarding device110 and its local termination module 230. Advertisements 260 and 262include reachability information indicating that the destinationaddresses within the subnet 20.0.0.0/24 are reachable though the serviceprovider network and specifically forwarding device 110. Thereachability information may have a next-hop IP address to reach20.0.0.0/24 as the IP address of forwarding device 110.

Suppose advertisement 260 reaches forwarding device 110 first. Localtermination module 230 stores the reachability information, or a hash ofthe reachability information, in a local storage and sends thereachability information onto customer routing device 204 in anadvertisement 264. Based on advertisement 264, customer routing device204 configures its routing table according to the reachabilityinformation. For example, customer routing device 204 may configure itsrouting table to route traffic addressed to the subnet 20.0.0.0/24 toforwarding device 110.

When advertisement 262 reaches forwarding device 110, local terminationmodule 230 compares advertisement 262's reachability information withthe local storage. It determines that advertisement 262's reachabilityinformation is stored in the local storage. And, accordingly, it doesnot forward the reachability information onto customer network 230.

FIG. 3 is a flowchart of a method 300 for masking redundant controllers,according to an embodiment.

Method 300 begins at step 302 when a customer routing device establishesa reachability information session with the edge forwarding device. Step302 may occur when the customer routing device and the edge forwardingdevice discover that they are physically connected to each other. Thereachability information session created at step 302 enables theforwarding device to exchange advertisements with the customer routingdevice. Advertisements from the customer routing device may indicatewhat addresses are reachable through the customer routing device on thecustomer network. And advertisements from the forwarding device mayindicate what addresses are reachable through the forwarding device onthe service provider network.

Once the customer routing device establishes the reachabilityinformation session at step 302, the edge forwarding device establishesreachability information sessions with each control device on theservice provider network at step 304. These reachability informationsessions enable the respective first and second control devices to sendadvertisements indicating what addresses are reachable through theservice provider network to the forwarding device.

Once the reachability information sessions are established, the routingdevices start exchanging reachability information. When the controldevices learn that a new address prefix is available through the serviceprovider network at decision block 312, each of the control devicesadvertise the prefix to the edge forwarding device at step 314. Then,the edge forwarding forwards one of the advertisements onto the customerrouting device at step 316 and discards the other. Steps 314 and 316 mayexecute to communicate all accessible prefixes when the session is firstestablished. Then, as illustrated in FIG. 3, steps 314 and 316 mayexecute incremental updates as the control devices learn that newprefixes are accessible.

Similarly, when the customer routing device learn that a new addressprefix is available through the customer network at decision block 306,the customer routing device advertises the prefix to the edge forwardingdevice at step 308. Then, the edge forwarding forwards the advertisementto each control device at step 310. Steps 308 and 310 may execute tocommunicate all accessible prefixes when the session is firstestablished. Then, as illustrated in FIG. 3, steps 308 and 310 mayexecute incremental updates as the customer routing device learns thatnew prefixes are accessible.

FIG. 4 is a diagram showing a system 400 that illustrates components ofthe local termination module 170 in greater detail. Local terminationmodule 170 includes three sub-modules: control session module 402,external session module 404, and exchange module 406.

Control session module 402 establishes the reachability informationsessions between forwarding device 108 and control devices 102 and 104as described above for example with respect to step 302. Externalsession module 404 establishes a reachability information sessionbetween the forwarding device and a customer routing device with respectto 304.

Exchange module 406 exchanges advertisements between the first andsecond reachability information sessions and the third reachabilityinformation session such that the first and second control device appearto the customer routing device to be a single device. Exchange module406 may send messages as illustrated in steps 306-316 in FIG. 3.

Each of the devices and modules disclosed herein may be implemented inhardware, software, firmware, or any combination thereof.

Each of the devices and modules disclosed herein may be implemented onthe same or different computing devices. Such computing devices caninclude, but are not limited to, a personal computer, a mobile devicesuch as a mobile phone, workstation, embedded system, game console,television, set-top box, or any other computing device. Further, acomputing device can include, but is not limited to, a device having aprocessor and memory, including a non-transitory memory, for executingand storing instructions. The memory may tangibly embody the data andprogram instructions. Software may include one or more applications andan operating system. Hardware can include, but is not limited to, aprocessor, a memory, and a graphical user interface display. Thecomputing device may also have multiple processors and multiple sharedor separate memory components. For example, the computing device may bea part of or the entirety of a clustered or distributed computingenvironment or server farm.

Identifiers, such as “(a),” “(b),” “(i),” “(ii),” etc., are sometimesused for different elements or steps. These identifiers are used forclarity and do not necessarily designate an order for the elements orsteps.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A method for exchanging routing data with anetwork including a separate control device and a plurality offorwarding devices, comprising: (a) establishing a first reachabilityinformation session between a forwarding device from the plurality offorwarding devices and a first control device such that the firstreachability information session enables the first control device tosend advertisements indicating what addresses are reachable through thenetwork, wherein the forwarding device is further part of: (i) a secondreachability information session established between the forwardingdevice and a second control device, the second reachability sessionenabling the second control device to send advertisements indicatingwhat addresses are reachable through the network, and (ii) a thirdreachability information session established between the forwardingdevice and an external routing device, the third reachabilityinformation session enabling the forwarding device to exchange theadvertisements with the external routing device indicating whataddresses are reachable through the respective forwarding and externalrouting devices; and (b) sending, by the first control device, a firstadvertisement to the forwarding device via the first reachabilityinformation session, the first advertisement comprising reachabilityinformation mapping destination addresses to next hop addresses, whereinthe forwarding device sends the first advertisement to the externalrouting device, (c) receiving, by the control device, externalreachability information, from the forwarding device, wherein theexternal reachability information was received by the forwarding devicefrom the external routing device via the first and second informationsessions to enable the first and second control devices to update howthey specify how the forwarding device is to forward data to theaddresses indicated in the external reachability information; (d)sending, by the first control device, a second advertisement to anotherforwarding device from the plurality of forwarding devices via a fourthreachability information session between the first control device andthe another forwarding device, the second advertisement including theexternal reachability information received in (c); (e) configuring, bythe first control device, at least one of a first routing table withinthe forwarding device and a second routing table within the anotherforwarding device according to network topology knowledge obtained overthe reachability sessions.
 2. The method of claim 1, further comprising:(f) exchanging, by the first control device, the first and secondadvertisements to the second control device using the forwarding deviceso that the first and second control device appear to the externalrouting device to be a single device.
 3. The method of claim 2, whereinthe forwarding device forwards data to an output port among a pluralityof output ports of the forwarding device based on a destination addressof the data or a label of the data using the routing table configured onthe forwarding device.
 4. The method of claim 1, wherein the first,second, and third reachability information sessions is a Border GatewayProtocol Control session.
 5. The method of claim 1, wherein at least oneof the first, second, and third reachability information sessions is anOpen Shortest Path First session.
 6. The method of claim 1, wherein thefirst and second control devices comprise the same network topologyknowledge.
 7. The method of claim 1, wherein at least one of the first,second, or third reachability information sessions is an IntermediateSystem to Intermediate System session.
 8. The method of claim 1, whereinthe forwarding device is further configured such that it sends the firstadvertisement received from the first control device to the externalrouting device only upon determining that the reachability informationwithin the first advertisement was not previously sent to the externalrouting device.
 9. A system for exchanging routing data with a networkincluding a separate control device and plurality of forwarding devices,comprising: a memory; and one or more processors coupled to the memoryand configured to: establish, by a control device, a first reachabilityinformation session between a forwarding device from the plurality offorwarding devices and the control device such that the firstreachability information session enables the control device to sendadvertisements indicating what addresses are reachable through thenetwork, wherein the forwarding device is further part of: (i) a secondreachability information session established between the forwardingdevice and a second control device, the second reachability sessionenabling the second control device to send advertisements indicatingwhat addresses are reachable through the network, and (ii) a thirdreachability information session established between the forwardingdevice and an external routing device, the third reachabilityinformation session enabling the forwarding device to exchange theadvertisements with the external routing device indicating whataddresses are reachable through the respective forwarding and externalrouting devices; and send, by the control device, a first advertisementto the forwarding device via the first reachability information session,the first advertisement comprising reachability information mappingdestination addresses to next hop addresses, wherein the forwardingdevice sends the first advertisement to the external routing device,receive, by the control device, external reachability information, fromthe forwarding device, wherein the external reachability information wasreceived by the forwarding device from the external routing device viathe first and second information sessions to enable the first and secondcontrol devices to update how they specify how the forwarding device isto forward data to the addresses indicated in the external reachabilityinformation; send, by the control device, a second advertisement toanother forwarding device from the plurality of forwarding devices via afourth reachability information session between the control device andthe another forwarding device, the second advertisement including theexternal reachability information received, configure, by the controldevice, at least one of a first routing table within the forwardingdevice and a second routing table within the another forwarding deviceaccording to network topology knowledge obtained over the reachabilitysessions.
 10. The system of claim 9, the one or more processors furtherconfigured to: exchange, by the control device, the first and secondadvertisements to the second control device using the forwarding deviceso that the first and second control device appear to the externalrouting device to be a single device.
 11. The system of claim 9, whereinthe forwarding device forwards data to an output port among a pluralityof output ports of the forwarding device based on a destination addressof the data or a label of the data using the routing table configured onthe forwarding device.
 12. The system of claim 9, wherein the first,second, and third reachability information sessions is a Border GatewayProtocol Control session.
 13. The system of claim 9, wherein the first,second, and third reachability information sessions is an Open ShortestPath First session.
 14. The system of claim 9, wherein at least one ofthe first, second, or third reachability information sessions is anIntermediate System to Intermediate System session.
 15. The system ofclaim 9, wherein the control device comprises the same network topologyknowledge as the second control devices.
 16. The system of claim 9,wherein the forwarding device is further configured such that it sendsthe first advertisement received from the control device to the externalrouting device only upon determining that the reachability informationwithin the first advertisement was not previously sent to the externalrouting device.
 17. A non-transitory program storage device tangiblyembodying a program of instructions executable by at least one machineto perform a method for exchanging routing data with a network includinga separate control device and plurality of forwarding devices, themethod comprising: (a) establishing a first reachability informationsession between a forwarding device from the plurality of forwardingdevices and a first control device such that the first reachabilityinformation session enables the first control device to sendadvertisements indicating what addresses are reachable through thenetwork, wherein the forwarding device is further part of: (i) a secondreachability information session established between the forwardingdevice and a second control device, the second reachability sessionenabling the second control device to send advertisements indicatingwhat addresses are reachable through the network, and (ii) a thirdreachability information session established between the forwardingdevice and an external routing device, the third reachabilityinformation session enabling the forwarding device to exchange theadvertisements with the external routing device indicating whataddresses are reachable through the respective forwarding and externalrouting devices; (b) sending, by the first control device, a firstadvertisement to the forwarding device via the first reachabilityinformation session, the first advertisement comprising reachabilityinformation mapping destination addresses to next hop addresses, whereinthe forwarding device sends the first advertisement to the externalrouting device; (c) receiving, by the control device, externalreachability information, from the forwarding device, wherein theexternal reachability information was received by the forwarding devicefrom the external routing device via the first and second informationsessions to enable the first and second control devices to update howthey specify how the forwarding device is to forward data to theaddresses indicated in the external reachability information; (d)sending, by the first control device, a second advertisement to anotherforwarding device from the plurality of forwarding devices via a fourthreachability information session between the first control device andthe another forwarding device, the second advertisement including theexternal reachability information received in (c); and (e) configuring,by the first control device, at least one of a first routing tablewithin the forwarding device and a second routing table within theanother forwarding device according to network topology knowledgeobtained over the reachability sessions.
 18. The non-transitory programstorage device of claim 17, the method further comprising: (f)exchanging, by the first control device, the first and secondadvertisements to the second control device using the forwarding deviceso that the first and second control device appear to the externalrouting device to be a single device.
 19. The non-transitory programstorage device of claim 17, wherein at least one of the first, second,and third reachability information sessions is an Open Shortest PathFirst session.
 20. The non-transitory program storage device of claim17, wherein at least one of the first, second, and third reachabilityinformation sessions is a Border Gateway Protocol Control session.